1. Field of the Invention
The present invention relates to improvement of a semiconductor device for switching of power. It also relates to a method and an apparatus for reducing a dead time to improve the accuracy and steadiness of controlling the order of low current and low voltage/low power which may affect the efficiency of inversion in a PWM (Pulse Width Modulation) converter composed of a circuit having two, upper and lower, arms.
2. Description of the Related Art
Such a PWM inverter as having two, upper and lower, arms is commonly used in a motor drive, an audio apparatus (a class-D power amplifier), or any other electrical appliances which include a pair of switching elements (e.g. transistors, FETS, IGBTS, or other semiconductor devices) connected in a series combination to a DC power source of 100 to 400 volts. In particular, the period for energizing the switching elements is controlled by on/off actions to provide a desired rate of voltage or current. However, if the two switching elements connected in series are simultaneously turned on, they are shortcircuited to the DC power source. Thus, it will receive an overcurrent resulting in breakdown.
It is known that a typical semiconductor switching element has an intrinsic time interval from the receipt of an on or off control signal to the starting up of its switching action. The action of the switching elements is normally faster for turning on than turning off. Also, the duration of action of a drive circuit depends on its characteristics. Accordingly, when one of the two switching elements in the upper or lower arm is turned off and simultaneously, the other switching element is turned on without giving a delay (a dead time), an instantaneous shortcircuit may be produced. To avoid such a shortcircuit, the drive circuit for the switching elements is preset unconditionally and perpetually with a dead time (a safety waiting time) of 3 to 6 microseconds which is determined in consideration of differences between the delays which are intrinsic to the elements and circuits and which are used for disabling the two switching elements. More specifically, since a conventional method and an apparatus for setting the dead time unconditionally and perpetually have to take into account variations in the operating time of the switching elements and in the temperature of use, the dead time is set to a considerable length, hence, impairing the linear control under low power ranges.
In the conventional method, it is desired to set the dead time to a length longer than a maximum time (toffmax) of the switching period of the switching element unconditionally and perpetually. For control under a very small power, the switching element has to be turned on and off at the shortest possible interval. A pulse width modulated (PWM) signal may be concealed within the dead time and fail to carry out a control action thus causing non-linear operation under the very small power. The non-linear operation will generate unwanted high-frequency noise in the output of the circuit.